This invention relates to thermal batteries employing a depolarized cathode comprising a major amount of cathode active material such as iron disulfide (FeS.sub.2).
A thermal battery is a battery which has an electrolyte comprising a salt mixture. Ordinarily, the salt mixture electrolyte is solid and chemically inert at room temperature, but when fused becomes fluid and highly conductive.
The development of thermal battery technology is discussed in detail in a report published by The American Society of Mechanical Engineers entitled A Review of Thermal Battery Technology by B. H. Van Domelen and R. D. Wehrle, reprinted from the 9th Intersociety Energy Conversion Engineering Conference, and is incorporated by reference herein.
Present day Ca/CaCrO.sub.4 thermal batteries have what is known as a "pellet" type construction, which is typically of a three-layer design comprising a heat pellet, an anode, and a DEB pellet sandwiched between the latter two described pellets.
The first "pellet" type thermal batteries included a heat pellet composed generally of powdered iron fuel and potassium perchlorate oxidizer pressed into a homogeneous pellet. The anode generally comprised either sheet calcium mechanically attached to a non-reactive metal substrate, or calcium metal vapor deposited upon a metal substrate. The substrate served both as a current collector and as a cell divider that prevented the calcium from reacting with an adjacent heat pellet of a three pellet single cell, and the cells were combined with end heat pellets and buffer pellets arranged to form a battery stack.
In the early pellet type thermal batteries the cathode (or DEB) pellet was generally composed of a mixture of depolarizer (conventionally known as the active cathode material which is reduced electrochemically during battery discharge), electrolyte, and binder. The depolarizer or cathode material commonly used was CaCrO.sub.4, the electrolyte was a LiCl-KCl eutectic, and the binder was a finely divided silica powder whose high surface area prevented the molten electrolyte from flowing. The materials were fused, ground and blended to obtain a homogeneous powder which permitted the pellet to be formed in one pressing.
Although CaCrO.sub.4 cells operate satisfactorily for most applications, recent work has shown that thermal cells using FeS.sub.2 as the depolarizer or cathode material provide more easily reproducible batteries and greater current capabilities, particularly on long-life tests, than do the CaCrO.sub.4 cells. The FeS.sub.2 type thermal cell requires a separator pellet (or a separator layer in a two-layer pellet) between the cathode and the anode, and is discussed in greater detail in Development of a Lithium Alloy Iron Disulfide 60-Minute Primary Thermal Battery, SAND 79-0814, Sandia Laboratories, April 1979, which is also incorporated by reference herein.
Although exhibiting longer life and greater voltage stability, these type cells pose a problem in that when activated, there is observed an abnormally high initial peak voltage, i.e., spike, which drops rapidly to the normal voltage output plateau when the cell is discharged at low-current densities. This poses a serious problem since in most applications voltage regulation is critical, and end of life of the battery is calculated as a percent value of the peak voltage (usually 75 or 80%). Thus, much useful life is wasted unless the spike is minimized. As a result, there have been various attempts to minimize and isolate the cause of the voltage spike, all of which have proven generally unsatisfactory. (For a detailed report see the report published by Sandia Laboratories, April 1979 entitled Studies of the Abnormally High Peak Voltage Observed with FeS.sub.2 Depolarized Thermal Batteries, also incorporated by reference herein).
One approach to eliminating or reducing the voltage spike discussed in said document involves treating the FeS.sub.2 with H.sub.2 S prior to manufacturing the cathode pellets. In the manufacturing of the pellets it is believed that the FeS.sub.2 reacts with atmospheric oxygen to form impurities which contribute to the cause of the spike. By treating the FeS.sub.2 with H.sub.2 S, the H.sub.2 S reacts with the surface oxidant on the FeS.sub.2 so that gaseous products are formed and carried away to reduce the surface to FeS.sub.2 alone.
Another approach involved adding small amounts of magnesium to the cathode pellet. Although the voltage spike was reduced, this approach did not completely remove the voltage spike, i.e., a spike of 1.241.+-.0.004 volts was still observed for a 28 volt battery. Other metals were also tried and deposited in a thin film on the cathode collector. The metals tested, which are above silver in the Electromotive Series, were also found to reduce the magnitude of the initial peak voltage. However, the lowest peak voltage observed in this technique was achieved with Zinc and still resulted in a peak voltage of 1.122 V. Thus, the lifetime of the battery to 80% peak voltage was still very short.
In a remotely related development, U.S. Pat. No. 4,163,829 discusses the concept of adding an anodic metal to an FeS.sub.2 cathode in a non-aqueous cell to reduce initial voltage spikes. However, this type of cell differs from the thermal cells to which the invention is applied and the results achieved in thermal cells with the materials suggested are worse than those achieved in conventional thermal cells. Thus they are not applicable for use in thermal type batteries.